The Ground Response Curve, Pillar Loading and Pillar Failure G.S. (Essie) Esterhuizen Chris Mark Michael Murphy The findings and conclusions in this presentation are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.

Background Review of retreat pillar mining Role of overburden response in loading and failure of pillars Gob

Overburden response Pillar loading Pillar failure

Overburden response

Stiff strong overburden Weak-soft overburden Overburden response can determine whether pillars fail or not Well developed pressure arch Poorly developed pressure arch

Weak Overburden behavior can be represented by a “ground response curve” Strong Load transferred to abutments by arching Load carried by internal supports Load transferred to abutments by arching

Ground response curves can be estimated using realistic numerical models Extraction panels Coal bed 3500 ft 2500 ft Strong beds

Ground response development conditions Ground response recorded at center of panel

2,000 1,500 1, Stress (psi) Overburden stress Convergence (inches) Width 80 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

2,000 1,500 1, Stress (psi) Overburden stress Convergence (inches) Width 80 ft Width 150 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

2,000 1,500 1, Stress (psi) Overburden stress Convergence (inches) Width 80 ft Width 150 ft Width 250 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

2,000 1,500 1, Stress (psi) Overburden stress Convergence (inches) Width 80 ft Width 150 ft Width 250 ft Width 500 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

2,000 1,500 1, Stress (psi) Overburden stress Convergence (inches) Width 1000 ft Width 80 ft Width 150 ft Width 250 ft Width 500 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

Ground response at the pillar extraction line Ground response was recorded at center of extraction line Gob Extraction line

Ground response at the pillar extraction line Panel width 500 ft at 1500 ft depth – Western US geology Stress (MPa) Convergence (m) At extraction line Development conditions 6,000 4,500 3,000 1,500 0 Stress (psi) Convergence (inches) Overburden stress

Pillar response

Pillar response curves Distributed stress over entire panel

Combined pillar and ground response curves Pillar W:H = 8 Overburden stress

Effect of panel width on pillar loading Pillar W:H = 8 Overburden stress

Effect of panel width on pillar loading Pillar W:H = 8

Effect of panel width on pillar loading Pillar W:H = 6 Overburden stress

Effect of panel width on pillar loading Yield pillar W:H = 3 Overburden stress 80 ft

Extraction line Development Pillar loading at the extraction line 500 ft wide panel at 1500 ft depth Western US geology

Extraction line Development

Pillar loading at the extraction line 500 ft wide panel at 1500 ft depth Western US geology Extraction line Development

Pillar loading at the extraction line 500 ft wide panel at 1500 ft depth Western US geology Extraction line Development

Pillar failure

Structural failure of pillars %2.0%4.0%6.0%8.0%10.0% Stress (MPa) Convergence W:H = 6 Structural strength = peak resistance Structural failure

Functional failure of pillars Functional failure Convergence

Conclusions Ground response and pressure arch formation are affected by: –Rock stiffness and strength –Panel width Ground response affects: –Pillar loading –Ultimate success/failure of pillars Ground response evaluation has demonstrated: –Importance of panel width on success of pillar mining –Structural failure of pillars does not necessarily imply functional failure –Functional failure appears to be related to ultimate pillar strain (%conv)